The invention relates to X-ray apparatus for radiologically examining a subject in which the beam emitted by an X-ray tube or like source is partially obstructed by a moving X-ray opaque mask, typically a rotating disk with a slit or other X-ray window which transmits a shaped scanning X-ray beam through the subject to a secondary image generating X-ray receptor such as a scintillation screen. As shown in U.S. Pat. Nos. 3,780,291 and 4,315,146, such equipment scans the subject with a nearly one dimensional, fan-shaped, beam rather than the full two dimensional pyramidal or conical beam radiating from the X-ray tube and a second rotating disk, coordinated with the first, masks radiation beyond the subject and therefore reduces the amount of scattered radiation in the image of the receptor. By scattered radiation is meant radiation incident on the X-ray receptor on paths other than directly from the source. Such scattered radiation degrades the contrast of the secondary image produced by direct radiation more or less depending on the fraction of scattered X-radiation relative to the total X-radiation on the receptor. An early form of multiple-masked scanner is shown in French Pat. No. 521,746 issued Mar. 14, 1921 to A. Dauvillier. Such early systems, however, required exposures times so long, 3 to 15 seconds, as to be of little use in modern rapid, multiple exposure X-ray examination of medical subjects.
A rapid sequence of X-ray exposures may be taken with scanning X-ray apparatus by increasing the power applied to the X-ray tube and hence the intensity of its emission. But there is a limit or maximum rated power which may be applied to an X-ray tube, and there is an upper limit to the fraction of scattered radiation which can be permitted without seriously degrading the quality of the secondary image.
Accordingly it is an object of the present invention to minimize the fraction of scattered radiation reaching the X-ray receptor while operating the X-ray tube within its power capacity.